Gas compressor with pulsation absorber for reducing cylinder nozzle resonant pulsation

ABSTRACT

A method for reducing pulsation effects associated with the compressor nozzle of a positive displacement compressor system. A pulsation absorber, having a design like that of a side branch absorber, is installed on the cylinder valve cap or on the cylinder nozzle. The acoustic dimensions and placement of the pulsation absorber are designed to reduce the amplitude of the pulsations associated with the peak resonant frequency of the compressor nozzle response.

TECHNICAL FIELD OF THE INVENTION

This invention relates to large positive displacement compressors fortransporting fluids, and more particularly to an improved method forreducing cylinder nozzle pulsations in large positive displacementcompressors.

BACKGROUND OF THE INVENTION

Most natural gas consumed in the United States is not produced close towhere it is used. To transport gas from increasingly remote productionsites to consumers, pipeline companies operate and maintain hundreds ofthousands of miles of natural gas transmission lines. The gas is thensold to local distribution companies, who deliver it to consumers, usinga network of more than a million miles of local distribution lines. Thisvast underground transmission and distribution system is capable ofmoving billions of cubic feet of gas each day.

To provide force to move the gas, operators install large gascompressors at transport stations along the pipelines. Reciprocatingcompressors are a type of positive displacement compressor that compressgas by using a piston in a cylinder and a back-and-forth motion. Asuction valve in the cylinder receives input gas, which is compressed,and discharged through a discharge valve. Reciprocating compressorsinherently generate transient pulsating flows, and various devices andcontrol methods have been developed to control these pulsations. Aproper pulsation control design reduces system pulsations to acceptablelevels without compromising compressor performance.

The state of the art in pulsation design and control technology hasevolved as compressor technology has changed. Designs for low-speedcompressors are more mature, with fewer critical issues. However,relatively recent high-speed, high-horsepower compressor designs areplacing significant challenges on pulsation control design.

Cylinder nozzle pulsations are one challenge to high-horsepower,high-speed, variable-speed units. The cylinder nozzle is the section ofpipe that connects the cylinder to the suction or discharge side of thecompressor, typically to a filter bottle. This section of pipe canprovide significant resonance responses.

Currently, one solution to attenuating cylinder nozzle pulsations is theinstallation of an orifice in the cylinder nozzle. For example, a platewith a flow restricting hole may be placed across the circumference ofthe nozzle. However, a downside of the orifice is that it causes apressure drop that requires the supply of additional horsepower. Thisburden can be significant on large horsepower units.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present embodiments and advantagesthereof may be acquired by referring to the following description takenin conjunction with the accompanying drawings, in which like referencenumbers indicate like features, and wherein:

FIG. 1 illustrates an integrated (low speed) compressor system.

FIG. 2 illustrates a separable (low or high speed) compressor system.

FIGS. 3A and 3B illustrate a nozzle pulsation absorber installed atalternative locations.

FIG. 4 illustrates test data of cylinder nozzle pulsations without anozzle pulsation absorber.

FIG. 5 illustrates test data of cylinder nozzle pulsations with a nozzlepulsation absorber installed on the valve cap.

FIG. 6 illustrates test data of cylinder nozzle pulsations with a nozzlepulsation absorber installed on the cylinder nozzle.

FIGS. 7-9 illustrate how the cylinder nozzle resonance is altered whenthe pulsation absorber is installed near the compressor valves.

DETAILED DESCRIPTION OF THE INVENTION

As explained in the Background, large reciprocating compressors used inthe gas and processing industries generate cylinder nozzle pulsationsthat can cause poor compressor performance, poor valve life, andsignificant vibration issues. The conventional approach to reducingnozzle pulsation is the installation of an orifice.

The following description is directed to a “nozzle pulsation absorber”which when properly designed, can significantly reduce the cylindernozzle resonant pulsations. The nozzle pulsation absorber can absorbcylinder nozzle resonant pulsations such that the maximum pulsations aredrastically reduced, eliminating the need for an orifice. Unlike aconventional orifice, the nozzle pulsation absorber does requireadditional horsepower.

Similar to the conventional orifice, the nozzle pulsation absorber canbe easily installed on an existing system at valve up. It can also beinstalled in the cylinder nozzle near the cylinder flange, but forexisting systems this is a more costly alternative.

FIG. 1 illustrates a reciprocating gas compressor system 100. Compressorsystem 100 is an “integrated” compressor system in the sense that itsengine 11 and compressor 12 share the same crankshaft 13. The engine 11is represented by three engine cylinders 11 a-11 c. Typically, engine 11is a two-stroke engine. The compressor 12 is represented by fourcompressor cylinders 12 a-12 d. In practice, engine 11 and compressor 12may each have fewer or more cylinders.

FIG. 2 illustrates a reciprocating gas compressor system 200 in whichthe engine 21 and compressor 22 are separate units. Thisengine/compressor configuration is referred to in the natural gasindustry as a “separable” compressor system. The respective crankshafts23 of engine 21 and compressor 22 are mechanically joined at a coupling24, which permits engine 21 to drive the compressor 22.

As indicated in the Background, a typical application of gas compressorsystems 100 and 200 is in the gas transmission industry. System 100 istypically a low speed system, whereas system 200 can be a low or highspeed system. The trend in the last decade is toward separable (highspeed) systems, which have a smaller footprint and permit coupling toeither an engine or electric motor.

Both systems 100 and 200 are characterized by having a reciprocatingcompressor 12 or 22, which has one or more internal combustioncylinders. Both systems have a controller 17 for control of parametersaffecting compressor load and capacity. Both systems can exhibit theresidual frequency problems discussed above.

As shown in FIG. 1, the compressor systems operate between two gastransmission lines. A first line, at a certain pressure, is referred toas the suction line. A second line, at a higher pressure, is referred toas the discharge line. Typically, the suction pressure and dischargepressure are measured in psi (pounds per square inch).

The following description is written in terms of the separable system200. However, the same concepts are applicable to system 100; asindicated in FIGS. 1 and 2, the same controller 17 may be used witheither type of system.

FIGS. 3A and 3B illustrate a nozzle pulsation absorber 30 installed atalternative locations, namely, the cylinder valve cap 32 (FIG. 3A) andthe cylinder nozzle 35 (FIG. 3B). Only a single cylinder 31 isrepresented, shown as an elevation view toward its valve cap 32.Cylinder 31 could be one of the cylinders from either system shown inFIG. 1 or FIG. 2.

The cylinder nozzle 35 is the section of pipe that connects the cylinder31 to the discharge or suction side of the compressor. In the embodimentof FIGS. 3A and 3B, the cylinder nozzle 35 is labeled on the dischargeside of the compressor, and the nozzle pulsation absorber 30 is on thedischarge side of cylinder 31. However, the nozzle pulsation absorber 30may be on either the suction or discharge side of the cylinder 31.

In the embodiments of FIGS. 3A and 3B, the cylinder 31 is connected tofilter bottles 33 and 34 at both the suction input and discharge outlet.These filter bottles 33 and 34 are installed as a common method forpulsation control, and are placed between the compressor and theattached piping systems. These filter bottles 33 and 34 operate withsurge volumes, and are commonly implemented as volume-choke-volumedevices. They function as low-pass acoustic filters, and attenuatepulsations on the basis of a predetermined Helmholtz response.

Each nozzle pulsation absorber 30 operates like a side branch absorber,and has a choke tube 30 a and surge volume 30 b. Choke tube 30 a is aspan of piping connecting the valve cap 32 or cylinder nozzle 35 to thesurge volume 30 b. In accordance with the invention, nozzle pulsationabsorber 30 reduces pulsations by altering the frequency of theresponses in the cylinder nozzle 35.

As is known in the art of side branch absorbers (also known as Helmholtzresonators) for other applications, the physical dimensions of choketube 30 a and surge volume 30 b are not the same as their acousticdimensions. The desired acoustic dimensions and the resulting physicaldimensions are determined by various known calculation and acousticmodeling techniques.

The acoustic dimensions of pulsation absorber 30 vary depending on thepulsation frequency to be dampened. The resonant frequency to be dampedmay be determined by various measurement or predictive techniques.

The connecting piping 30 a is attached to the valve cap 32 or nozzle 35,such that pulsations corresponding to the acoustic natural frequency ofthe pulsation absorber 30 are absorbed from the compressor system. Thediameter and size of the connecting piping 30 a and the size of thesurge volume 30 b determine the acoustic natural frequency of thepulsation absorber 30.

Advantages of the above-described nozzle pulsation absorber 30 are thatit controls cylinder nozzle pulsations, with significant reduction ofpeak pulsation amplitudes and pulsations at resonance. Its design issimple, and it is easy to install on an existing system.

FIG. 4 illustrates test data from a compressor having a 8.5 inch boreand 3 inch stroke, running at 500 to 1000 rpm, without a nozzlepulsation absorber. As illustrated, the cylinder nozzle response has asignificant peak at approximately 50 Hz.

FIG. 5 illustrates test data from the same compressor, under the sameoperating conditions as FIG. 4, but with a nozzle pulsation absorber 30installed on the valve cap 32. As illustrated, the cylinder nozzleresponse is split, to approximately 43 and 58 Hz.

FIG. 6 illustrates test data from the same compressor, under the sameoperating conditions as FIG. 4, but with a nozzle pulsation absorber 30installed on the cylinder nozzle 35. As illustrated, the cylinder nozzleresponse is split, to approximately 42 and 56 Hz.

FIGS. 7-9 illustrate how the cylinder nozzle resonance is altered whenthe pulsation absorber is installed near the compressor valves. FIG. 7depicts the velocity profile that is typically associated with acylinder nozzle resonance. FIG. 8 depicts the velocity profile that istypically associated with a side branch resonator. As shown in FIG. 9,by installing the pulsation absorber near the compressor valves, a gasvelocity “maximum” is generated at the location where a velocity“minimum” would typically form when the pulsation absorber is notinstalled.

1. A method of reducing pulsations associated with the compressor nozzleof a positive displacement compressor, comprising: determining theresonant frequency of the pulsations; placing a side branch absorber onthe cylinder valve cap; wherein the side branch absorber has a surgevolume and a choke tube; wherein the side branch absorber has thefollowing dimensions operable to reduce the peak pulsation amplitude ofthe resonant frequency: volume of the surge volume, length of the choketube, and diameter of the choke tube.
 2. The method of claim 1, whereinthe side branch absorber has an acoustic frequency that splits thecylinder nozzle frequency response.
 3. The method of claim 1, whereinthe pulsation absorber is placed at a location such that a gas velocity“maximum” is generated at the location where a velocity “minimum” wouldtypically form when the pulsation absorber is not installed.
 4. A methodof reducing pulsations associated with the compressor nozzle of apositive displacement compressor, comprising: measuring the resonantfrequency of the pulsations; placing a side branch absorber on thecylinder nozzle; wherein the side branch absorber has a surge volume anda choke tube; wherein the side branch absorber has the followingdimensions operable to reduce the peak pulsation amplitude of theresonant frequency: volume of the surge volume, length of the choketube, and diameter of the choke tube.
 5. The method of claim 4, whereinthe side branch absorber has an acoustic frequency that splits thecylinder nozzle frequency response.
 6. The method of claim 4, whereinthe pulsation absorber is placed at a location such that a gas velocity“maximum” is generated at the location where a velocity “minimum” wouldtypically form when the pulsation absorber is not installed.